[minimax]

When you are programmatically trying to unscramble something that has been encrypted, you end up trying a bunch of different keys. When you try a key and you want to know if the unscrambling worked, you can check the letter frequency distribution in your result against the letter frequency distribution of a big English corpus. If the distributions are close, you can be more confident that your key is correct.

It actually works pretty well. You end up having to do something like this pretty early on in the Matasano crypto challenge problems.